In modern hydraulic systems, three position spool valves are commonly employed to control the operation of various types of large oftentimes complex equipment. The activating forces (e.g., push, pull) for adjusting such a valve from a normal rest position are frequently applied by a lever or the like connected to the valve spool.
The main force to be overcome in activating a valve in either direction is the force of the spool centering spring, the function of which is to provide enough force in the closing direction to insure that the valve spool will return to a normal rest (closed) position any time the externally applied actuation force is removed. The common term for this valve characteristic is "dead man control."
The resistance to spool movement is comparatively low in most instances, except in a very narrow range of spool movement which occurs just before the spool closing position is reached wherein the dynamics of high velocity fluid flow within the valve creates a high force tending to resist closing. A high spring force from the spool biasing spring is needed only during this range of movement; however, to achieve this result, a high strength spring will be utilized which offers high resistance to the spool actuation at all points of the actuation stroke. As valves increase in flow capacity this momentary resistance to closing increases and requires the use of stronger centering springs which in turn raise the input forces required for spool actuations. High actuation forces produce serious operator fatigue and at the same time make it more difficult to control small increments of lever motion particularly where careful throttling action is desired.
Heretofore, in an effort to resolve the operator fatigue problems due to the high resistance of the spool biasing spring, various complex and costly pressure compensated valve designs were utilized which enabled moderate size biasing springs to be incorporated in the valve structure. In addition, various force multipliers, or pressure pilot operated control systems, or electrical proportional control systems, were sometimes employed, which in addition to being costly were highly susceptible to malfunction or unreliability due to climatic changes, and required an inordinate amount of maintenance and repair. Furthermore, where the valve required only light input forces to effect acutation thereof, the stability of the valve to remain in its rest, or neutral, position was impaired because of its inability to withstand input forces due to inadvertent bumping by the operator, jarring due to terrain configuration, or due to possible internal pressure imbalances.